A battery cell generates a maximum voltage across its terminals when the battery cell is fully charged. During use, in which power is drawn from the battery, the battery voltage drops gradually until the battery cell reaches a point in its discharge cycle wherein the battery voltage begins to drop off more rapidly and the cell becomes depleted. Under equal load conditions, the rate at which a battery cell loses its charge varies depending upon the materials from which the battery cell is constructed and also may vary from one battery cell to another battery cell of the same type.
The voltage level to which a battery cell of a given type may be charged also depends upon the materials from which the battery cell is constructed. For example, a NiCad battery cell can be charged to approximately 1.2 volts. Because the voltage (and current) that may be developed by a single battery cell is limited, many battery powered devices that require more power than a single battery cell can provide utilize a stack of battery cells as a power source. A battery cell stack is constructed by stacking a number of battery cells in series and obtaining power from the stack by connecting a terminal to each of the outermost terminals of the battery cells at each end of the stack. For example, if a given device requires a voltage level of approximately 6.0 volts to operate, a battery stack comprising five NiCad batteries (in series), which will generate approximately 6.0 volts, may be used to power the device. Alternatively, a battery stack comprising some other number of cells may be regulated to the required voltage level by a power converter circuit.
If one of the battery cells in the stack is depleted of charge to a certain degree before other cells in the stack, the depleted cell may become damaged by a reversed polarity voltage imposed upon the depleted cell which is caused by the current which travels through all of the cells of the stack. This resulting damage may affect the ability of the damaged cell to be recharged effectively at a later time. This, in turn, may exacerbate the problem by causing the damaged cell to again become depleted earlier than other cells of the stack in a subsequent discharge cycle.
One solution to this problem is to avoid depleting the entire battery cell stack to a point where a single cell may become subjected to the above-described damage. This solution, however, has drawbacks. One drawback is that it is not possible to predict precisely when this point will be reached, having the result that a significant amount of usable battery power may be remaining in the stack when the stack is recharged. Therefore, the battery stack will have to be recharged more often which is, at least, inconvenient. Another drawback is that some types of rechargeable batteries suffer from a problem known as "memory" wherein a battery cell that is not fully discharged will not be able to become recharged to a capacity that is as great as if the cell had been fully discharged before charging. Therefore, the total charge capacity of the stack may also be diminished.
What is needed, therefore, is a battery power conversion circuit that overcomes the above-mentioned drawbacks.